It is known from EP-A 0 100 119, EP-A 1 221 442, DE-A 199 54 322 and EP-A 0 904 151 that olefins can be reacted with hydrogen peroxide to give an epoxide when a purely inorganic titanium-containing zeolite is used as catalyst.
However, all these disclosed catalysts have the disadvantage that the oxidizing agent being used (hydrogen peroxide; ethyl- or isopropylbenzene hydroperoxide) decomposes to some extent on these catalysts. The consequences are epoxide yields, with respect to the oxidizing agent, of <100% and in some cases safety-engineering problems due to the formation of molecular oxygen as a decomposition product of the oxidizing agent.
Furthermore, all the disclosed catalysts have the disadvantage that they progressively lose their catalytic activity during the reaction.
The disclosure in WO 99/01445 keeps the desired minimum olefin conversion constant for a limited time by increasing the reaction temperature and/or pressure. The technical limits, however, are very restricted due to the high epoxide reactivity. Even small increases in temperature can markedly reduce the epoxide selectivity. In industrial plants operating on the kiloton scale, small reductions in product selectivity can endanger economic viability.
EP-A 0 743 094 and EP-A 0 790 075 describe thermal regeneration, preferably with molecular oxygen. To achieve target temperatures of 200, better 550° C., in a few cases the catalyst has to be removed from the reactor. At least, it is a common factor in the disclosed regeneration processes that the epoxidation reaction has to be interrupted for the regeneration period.
Short catalyst operating lifetimes result in production losses during the regeneration phase or require a redundant, cost-intensive production pathway. Thus, the development of new catalysts which can achieve high activities with industrially interesting operating lifetimes and high selectivities is desirable.
EP-A 1 221 442 describes regeneration of the titanium zeolite catalyst TS1 with aqueous hydrogen peroxide. The disclosure is characterized in particular in that regeneration can be performed while the epoxide reaction is taking place in a continuous flow system that is in the presence of olefin, methanol and aqueous hydrogen peroxide.
The mechanism of deactivation is not fully understood. Possibly, coating of the catalytically active solid surfaces with organic molecules takes place to such an extent that the active epoxidizing species is no longer available for the desired reaction.
DE-A 199 54 322 describes TS1 molded catalysts which are characterized in that the extrudates are composed of TS1 powder and other materials based on SiO2 for molding purposes. These extrudates, which thus contain crystalline and non-crystalline SiO2 phases, are impregnated with aminopropyltrialkoxysilane and a base, and simultaneously a reagent to modify the surface of the extrudate, and then calcined at 550° C. in a stream of air until no more silicon-carbon bonds can be analytically detected. Although the purely inorganic molded catalysts obtained in this way have the same catalytic activity as similar systems without any silane surface treatment in a reaction step which follows TS1 synthesis, they have the tendency to generate slightly fewer secondary products. In addition the resistance of the strands of extrudate to lateral pressure is about 50% higher. The subsequent reaction of PO with water or methanol to give propylene glycol and methoxypropanol respectively is obviously suppressed a little as compared with disclosed catalysts.
The data in the table given below demonstrate this:
without modificationwith modificationmethoxypropanol [ppm]:3100-38001700-3200propanediol [ppm]:400-600300-500
For an industrial process, the development of catalysts which achieve longer catalyst operating lifetimes along with still higher epoxide selectivities and epoxide productivities is desirable. Furthermore, it would be desirable to waste less of the expensive oxidizing agent due to decomposition on the catalyst and for use during catalyst regeneration.
To prepare catalysts on an industrial scale (ton scale) the process steps for catalyst preparation should be as reproducible and simple as possible. In order to achieve an economically viable process, the costs of catalyst preparation should be very low.